In the field of vehicle seat belt apparatus, there has been used in recent years a technique for, in an emergency or during unstable travel of the vehicle, holding or restraining a vehicle occupant to a seat with a seat belt to thereby limit a posture change of the vehicle occupant. However, in the conventional seat belt apparatus based on such a technique, which are mainly intended to protectively restrain the vehicle occupant to a seat and support the vehicle occupant against a posture change, the vehicle-occupant holding or restraining force would sometimes become excessively great, and there is a need to provide a separate tension sensor for detecting tension of the belt with a view to optimizing the vehicle-occupant restraining force. Further, in a case where control is performed to produce predetermined belt tension, the conventional seat belt apparatus can not restrain a vehicle occupant stably with a small force. Because frequent activation of the seat belt apparatus (i.e., vehicle-occupant restraining operation) increases a feeling of discomfort of the vehicle occupant, it has been necessary to limit a frequency of activation of the vehicle-occupant restraining operation, for example, by activating the vehicle-occupant restraining operation only in emergent situations and preventing the vehicle-occupant restraining operation from being activated during normal traveling of the vehicle.
Among various prior art literatures disclosing such vehicle seat belt apparatus are Japanese Patent Application Laid-Open Publication Nos. 2004-291967 (hereinafter “Patent Literature 1”) and 2001-122081 (hereinafter “Patent Literature 2”). Namely, Patent Literature 1 discloses a vehicle-occupant restraining/protecting system which is arranged to protectively restrain a vehicle occupant with an enhanced efficiency by controlling the restraining operation with a condition of the vehicle taken into account. Patent Literature 2 discloses a vehicle-occupant restraining/protecting system which is arranged to protect a vehicle occupant with an enhanced efficiency by activating a pretensioner of the seat belt apparatus, for example, when the vehicle body is slipping sideways or the driven wheels are running idle during travel of the vehicle.
However, the conventional vehicle seat belt apparatus can not meet both the request that a posture change of a vehicle occupant be limited by the vehicle-occupant restraining operation in an emergency and the request that comfort of the vehicle occupant be enhanced by properly controlling production of the vehicle-occupant restraining force. Particularly, once the belt tension suddenly changes due to a posture change of the vehicle occupant or the like, the conventional vehicle seat belt apparatus tend to produce belt tension uncomfortable for the vehicle occupant, in order to control the belt tension to a target tension level. Also, in a case where a posture change of the vehicle occupant occurs frequently, there would arise other inconveniences, such as slack produced in the belt or residual tension remaining in the belt to undesirably keep the belt taut.